Startability of engines fueled with gasoline during cold ambient conditions (also known as an engine cold-start) may be an issue due to the difficultly of evaporating sufficient fuel to provide a combustible air-fuel ratio. In particular, the larger amount of time required to evaporate the fuel may degrade engine startability. The issue may be exacerbated when the engine is fueled with alternate fuels, such as ethanol fuels (e.g., E85, E100, etc.). Therein, the additional charge cooling effect of the alcohol fuel can lower the intake aircharge temperature at cold-start conditions, further degrading combustion stability and increasing potential for engine misfire.
One example approach for improving engine startability during cold conditions is shown by Krengel et al. in U.S. Pat. No. 8,447,496. Therein, during an engine cold-start, at least a portion of fuel is direct injected during a compression stroke, and a remainder of the fuel is direct injected during the intake stroke. Further, the portion of fuel delivered as a compression stroke injection is increased as the alcohol content of the injected fuel increases. By delivering at least some fuel as a compression stroke injection, the higher air-charge temperature of the engine during the compression stroke is leveraged to improve fuel evaporation.
However, the inventors herein have recognized potential issues with such a system. As one example, even with fueling during the compression stroke, at low ambient conditions, there may not be sufficient charge temperature to evaporate the fuel for engine starting. The inventors herein have further recognized that, even when fueling is delivered during a compression stroke, the fuel evaporation is driven by difference in temperature between the boiling point of a fuel and an aircharge temperature during a compression stroke. However, the boiling point of the fuel is a function of the aircharge pressure during the compression stroke. If the fuel boiling point can be reduced while keeping the aircharge temperature the same, the evaporation of the fuel can be enhanced.
Thus, in one example, the issues described above may be addressed by a method for improving engine startability comprising: during an engine cold-start, determining a fuel injection including an amount of fuel and a timing of injection based on engine operating conditions; and lowering manifold pressure for a first combustion event in engine cylinder during the cold-start based on an estimate of fuel temperature at an end of the injection. Herein, the fuel injection includes a single compression stroke injection. In this way, fuel evaporation during a cold-start is improved.
As one example, on a first combustion event of each cylinder during an engine cold-start, fuel may be delivered as a single compression stroke injection. In addition, manifold pressure may be lowered for the first combustion event in each cylinder by reducing an intake throttle opening. Specifically, manifold pressure may be lowered to a value that is optimized based on the effect of the reduced manifold pressure on each of fuel boiling point and aircharge volume, the optimization enabling a combustion air-fuel ratio to be maintained at a target value (such as at or near stoichiometry). As such, when the manifold pressure is reduced, the compression pressure is also reduced. The reduced pressure reduces the boiling point of the fuel being delivered. Since the temperature of the aircharge during the compression stroke is independent of the manifold pressure, it remains the same, resulting in an isentropic compression stroke injection. The fuel temperature then changes during the compression stroke as a function of the initial temperature of the aircharge before compression. As a result, by reducing the boiling point via application of a lower manifold pressure while keeping the charge temperature the same, the evaporation of the fuel is enhanced. At the same time, the lowering of the manifold pressure reduces the volume of the aircharge in the cylinder, reducing the amount of fuel required to be evaporated to maintain a given air-fuel ratio. On a second combustion event of each engine cylinder, the manifold pressure may then be raised (e.g., to nominal levels). By iteratively optimizing a manifold pressure set-point based on the reduced need for fuel and the improved fuel evaporation at the lower manifold pressure, the target air-fuel ratio can be achieved more easily at the engine cold-start.
The technical effect of lowering the boiling point of a fuel during cold ambient conditions by lowering manifold pressure, while keeping the charge temperature the same, is that a larger difference between fuel boiling point and aircharge temperature is achieved. As a result, fuel may be effectively evaporated to form a combustible/stoichiometric air-fuel mixture. By optimizing the set-point to which the manifold pressure is lowered on a first combustion event in each engine cylinder (during an engine restart) based on a balance between a reduction in the amount of cylinder charge (and therefore a reduction in the amount of fuel required in the cylinder) and an enhancement in fuel evaporation, engine startability at colder temperatures can be improved, even when alcohol fuels are used, without degrading engine torque output, or exhaust emissions. Additionally, by evaporating most of the injected fuel, less fuel may be lost during engine operation, and the need for larger or pilot fuel injections at engine cold-start may be reduced or eliminated. As such, this may provide fuel economy benefits as well as reduced cold-start exhaust emissions. In addition, the occurrence of engine misfires during an engine cold-start are decreased. Further, by maintaining use of compression injection on the first combustion event of a cold-start, it is possible to maintain repeatable engine speed profiles.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.